This invention relates generally to noncontact temperature measuring and more particularly to improvements in infrared thermometers for minimizing errors associated with ambient temperature transients.
U.S. Pat. No. 4,456,390 issued to K.G. Junkert and H.P. Vosnick, and assigned to the assigned of the present application, discloses a portable noncontact temperature measuring instrument incorporating improved temperature compensating and signal processing circuitry. More specifically, the patent discloses an infrared thermometer which utilizes a thermopile detector to develop a temperature indicating output signal for driving a display. The thermometer includes a temperature sensor, (e.g. a diode) thermally coupled to the thermopile to compensate for temperature induced variations in the thermopile responsivity. Additionally, the instrument incorporates circuit improvements including means for compensating for amplifier drift.
Although the aforementioned improvements markedly enhance the instrument's overall performance by minimizing errors attributable to certain sources, nevertheless, errors attributable to rapid ambient temperature transients can still occur. Such errors are particularly troublesome in portable instruments which are typically used in a variety of industrial and energy oriented applications involving relatively harsh environments. More particularly, users typically subject such portable instruments to extreme and rapid ambient temperature changes as would be experienced when moving an instrument from a storage office at 72 degrees F ambient to a location proximate to an industrial furnace where the ambient may exceed 100 degrees F and be characterized by drafts of air flowing over the instrument.
More generally, it is not uncommon for users to subject such portable instruments to ambient temperatures varying from 20 degrees F. to 120 degrees F. and changing at a rate in excess of 1 degree per minute. Such ambient temperature changes typically produce an intrusion of heat into the sensing area or "hot junction" of the thermopile via thermal paths created by external thermopile terminals, housing elements, ambient air currents, etc. As a consequence, the output signal developed by the thermopile can include a component, attributable to the relatively large thermal transients piped into the hot junction via the aforementioned thermal paths, which effectively swamps the signal component attributable to the relatively low level radiation from the target.
Efforts have been made to exclude or compensate for the effect of these transients. For example, it has been suggested that a relatively large heat sink be associated with the thermopile to stabilize it against rapid ambient temperature changes (e.g., see, U.S. Pat. No. 4,301,682). The problem with this approach is that it slows the response time of the instrument and requires relatively long waiting periods to avoid erroneous readings. An alternative approach has relied on periodically interrupting the incoming radiation or temperature indicating signal to re-zero the instrument to null out the influence of ambient temperature. Although these approaches can prove helpful, they are generally insufficient to avoid the overshoot reading errors generally associated with large and rapid ambient temperature changes.